The flexible display has been considered for the display industry as the next generation of information display technology. Particularly, the flexible active-matrix organic light-emitting diode (AMOLED) display has attracted much interests in the applications such as smart phones, intelligent home systems, wearable electronic devices, etc.
Thin-film transistor (TFT) devices used in an active matrix may be degraded if moisture penetrates into the device layer. For example, moisture may cause the characteristics of a TFT device to deviate from designed range, resulting in a malfunctioned device. Further, organic light-emitting diode (OLED) devices are extremely sensitive to moisture and oxygen. For example, the emission of an OLED device may degrade when OLED materials are exposed to moisture. The highly reactive and low work function cathodes of an OLED device can be easily corroded by moisture and oxygen. Thus, AMOLED display generally needs two substrates to encapsulate the TFT and OLED devices to ensure stable performance of those devices.
Due to its flexibility to bend, plastic substrates have been selected to be the substrates for flexible display panels. In addition, plastic substrates are light weighted and not fragile, and are suitable for roll-to-roll processing. Display panels made with plastic substrates are proved to be durable to mechanical shocks and allow rollable and foldable applications. In some cases, the bending radius is required to be less than 1 mm.
However, plastic substrates are known to be less effective than glass substrates to prevent moisture from coming into the device layer. To combat these drawbacks, several proposals have been implemented. For example, referring to FIG. 1, a stacked layer 104 is provided on a plastic substrate 102 to reduce moisture and oxygen penetrating therethrough. Stacked layer 104 includes a plurality of pairs of stacked films. A pair of stacked films includes a polymer film 106 and an inorganic film 108. For example, in FIG. 1, three pairs of polymer films 106 and inorganic films 108 are provided on plastic substrate 102.
Generally, inorganic films 108 are formed on substrate 102 by sputtering, plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), etc. Polymer films 106 may be formed by evaporating monomers onto substrate 102 which are further cured by heat or light. However, due to the low heat resistibility of plastic substrate 102, the processing temperature of forming the polymer is limited to, typically, less than 150° C. Further, processing temperature of forming TFT device layer which is generally provided on stacked layer 104 is limited by the processing temperature of stacked layer 104. That is, a TFT device layer on a plastic substrate is generally formed at a lower temperature, for example, a temperature under 150° C. A TFT device formed at low temperature has insufficient device performance and device reliability, and it may not be suitable for controlling liquid crystal of a liquid crystal display or the OLED devices of an OLED display.
To solve the above-mentioned issue, a new structure of a stacked layer on top of a plastic substrate is proposed. Referring to FIG. 2, a stacked layer 204 is provided in top of a plastic substrate 202 to reduce moisture and oxygen penetrating therethrough. Stacked layer 204 may include a plurality of pairs of stacked films. A pair of stacked films includes a first inorganic film 206 and a second inorganic film 208. For example, in FIG. 2, three pairs of first inorganic film 206 and second inorganic film 208 are provided on plastic substrate 202. Inorganic films 206, 208 may be SiO2 and SiNx.
Although these inorganic films can withstand higher temperature to form a TFT device layer, they have drawbacks as well. Most importantly, particles and pin-holes are common during device manufacturing, and a single inorganic layer cannot cover particles or pin-hole completely. Thus, moisture or oxygen can penetrate along the boundary of pin-hole or particles. Typically, very thick inorganic barrier layers are required. Particularly, it may need several pairs of stacked inorganic films to obtain better film coverage and reduce moisture and oxygen penetration. As the films pile up and thickness increases, the internal stress builds up, which may cause the plastic substrate to bend and may compromise the barrier effect of the inorganic films. Moreover, the surface roughness becomes higher with more stacked inorganic films.
Therefore, there is a need to improve the structures of a flexible display to increase the reliability of the flexible display.